I had planned the third part be an extension of how the sinusoidal 3D-wedge model influenced eruptions over time and how it explains distal eruptions along the fissure swarm. But, there has been a wealth of questions and comments that I felt needed to be answered that do not belong to the article series itself, but are so important that they needed to be addressed in the series, even though they do not technically have anything to do with the proposed model
A clarification of the model
I noticed that a couple of people had a problem with envisioning the model in their heads, especially the part that it is open at the bottom. It seems that some thought I was talking about an intra-crustal wedge with a closed bottom. This was not the case.
Thankfully Andrej Flis has once more produced a specialized image of Hekla. This time it is the Moho based on earthquake data. It shows the wedge from an angle.
As you can see it is open at the bottom and follow a sinusoidal wedge. It is a bit wider than I had envisaged, so we will have to up both the length at the bottom and the volume of lava it contains. In retrospect, this makes the model fit better with known eruptive volumes.
Hekla on the Rocks
There was also a discussion about if Hekla has a glacier or not. The answer is that Hekla both has a glacier and that it does not have a glacier.
Hekla is by far tall enough to have a rather sound glacier, at the same time the edifice area is too small to have a large imposing glacier. This results in a small top glacier that will inevitably melt away during eruptions.
Before 1947 eruptions was more interspersed and therefore the glacier had more time to rebuild between eruptions. At the same time, at some point backwards in time the edifice was too low for a glacier to form, therefore there are only 3 glacial periods registered in the official records of Iceland as evidenced in records of jökulhlaups caused by Hekla eruptions.
These happened in 1766, 1845 and 1947. Between these eruptions there was written records of a growing glacier, there is also evidence in the form of paintings and photographs. It is also well recorded that there where jökulhlaups emanating from Hekla during these eruptions.
Nowadays the repose times are far shorter between Hekla eruptions, so there is almost no time for a glacier to regrow, but a few meters had time to mass between the 1947 and 1970 eruptions and there are indeed signs that a diminutive glacier started to grow back last summer.
The non-existing flood-basalts
In the comments after the last part of the series it was mentioned that Hekla had large basalt floods prior to onset of the large explosive eruptions. That is not at all true, this error stems from Global Volcanism Program, that for some reason have used an old article that mixes the list of true Heklugjá eruptions, fissure swarm eruptions of Hekla and in an utterly confusing way have also included the eruptions of the totally separate volcanic system of Vatnafjöll.
The large early basalt flows that the GVP lists are from Vatnafjöll and no other volcano. They are completely petrochemically different from any sample from Hekla, but are dead matches for Vatnafjöll.
In fact, the magmas of Hekla and Vatnafjöll are as distant from each other as Vatnafjöll and Etna. In a way, it would have been better that they had mixed Hekla and Etna up, they are more related in many respects. (I hope Boris Behncke didn’t read that sentence.)
The first known eruption of Hekla is the VEI-5 Hekla-5 tephra in 5 150BC. It is also well worth noting that no known effusive edifice building eruptions occurred during the first 5 500 years of Hekla’s existence. I will return to this in a later part of the article series.
A true list of Hekla eruptions
I feel that I should produce a list of Hekla eruptions that remove all eruptions that do not belong to Hekla and that separates the fissure swarm eruptions from the true Hekla eruptions that come from the central volcano itself.
I urge the GVP to change their list and use the list below, or the list proposed by Professor Erik Sturkell. After all, my list is based upon his list and I do not think that anyone would doubt Sturkell since he is the recognized theoretical master of the subject of Hekla.
The list will be from the first eruption to the last. Pre-edifice means that it was prior to the formation of the edifice of Hekla, proto-edifice means that it may be initial stages of edifice building. Edifice means that the eruption added to the edifice of the central volcano. Fissure means that it was an eruption on the fissure swarm on Hekla, these are always non-explosive basalt events that are different from the main eruptions of Hekla.
Year Name VEI Effusive Hekla/fissure
5 150BC H-5 Tephra 5 No Pre-edifice
4 700BC DH Tephra 4 No Pre-edifice
4 110BC Hekla Ö Tephra 5 No Pre-edifice
2 310BC H-4 Tephra 5 No Pre-edifice
1 550BC Seslund pumice 4 Unknown Proto-edifice?
1 100BC H-3 Tephra 5-6 No Proto-edifice?
650AD Hellkvislarhraun ? Yes Edifice
800AD Heklutaglahraun, Taglgigahraun, 1-2 Yes Fissure
1104AD H-1 Tephra 5 Yes Edifice
1158AD 4 Yes Edifice
1206AD 3 Yes Edifice
1222AD 2 Yes Edifice
1300AD 4 Yes Edifice
1341AD 3 Yes Edifice
1389AD Raudoldur 3 Yes Edifice
1510AD 4 Yes Edifice
1554AD Raudubjallar 2 Yes Fissure
1597AD 4 Yes Edifice
1636AD 3 Yes Edifice
1694AD 4 Yes Edifice
1725AD 1 Yes Fissure
1766AD Bjallagigar 4 Yes Edifice
1845AD 4 Yes Edifice
1878AD Krakagigar, possible Hekla 2 Yes Fissure?
1913AD Mundafit, Lambafit 2 Yes Fissure
1947AD Hraungigur, Axlargigur & 4 Yes Edifice
1970AD Sudurgigar, Hlidargigar & 3 Yes Edifice
1980AD 3 Yes Edifice
1981AD 2 Yes Edifice
1991AD 3 Yes Edifice
2000AD 3 Yes Edifice
This gives us a list of 30 Hekla eruptions that we can divide into 3 distinct categories. The first category consists of the first 6 highly explosive eruptions that are spaced over 4000 years. These eruptions did not in any large degree build an edifice, but they did originate from the Heklugjá fissure and consist of calk-alkali andesite of the Hekla type.
The second category consists of 5 fissure eruptions that occurred on the Hekla fissure swarm. They all consist of Hekla type basalts.
The third category are the main eruptions of Hekla. There are 21 of these between 650AD and today. There is a bit of an intriguing pattern among those since there seems to be 3 periods of heightened activity after larger than usual eruptions.
The first period is between 1104 to 1389AD and consists of 7 eruptions with an average interval of 41 years in between. The average eruptive strength was VEI-3.4.
The second period is between 1597 to 1766 and consists of 3 eruptions with an average interval of 32 years in between eruptions. The average eruptive strength was VEI-3.7.
The third and current period of heightened activity started in 1947 and is as far as we know continuing. So far it consists of 6 eruptions with an average rate in between of 9 years if we use 2000AD as a cut-off date for the last eruption. If we assume that there is an upcoming eruption the interval is now 12 years and counting. The average eruptive strength is VEI-3.
In the next instalment I will return to why the rate of eruptions are increasing and why they are becoming less violent over time, in other words, it is time to return to interpreting Hekla according to the new model.